1. Field of the Art
This invention relates to an arrangement for detecting a focus control signal in an optical head for recording information onto information recording medium, e.g., an optical card, or an optical disk, etc. and reproducing the same therefrom.
2. Prior Art
When information is recorded and/or reproduced using an optical head, a light beam is focused using a focusing lens while moving or rotating information recording medium (which will be simply referred to as "medium" hereinafter) in a plane direction, to thereby focus on the medium, thus to irradiate the light beam onto the medium.
In this instance, when the medium is moved or rotated, vertical plane vibrations are produced on the medium surface due to the curvature or warp of the medium itself, mechanical error of the moving/rotating mechanism, or the like.
Such plane vibrations ordinarily have an amplitude of approximately 100 microns. On the other hand, since the depth of a focus of the focusing lens is a small value of approximately several microns, a large deviation or discrepancy of focus occurs at all times at the time of recording/reproducing of information, with the result that the information is impossible to record or reproduce.
To prevent such a deviation of the focus, optical heads of various configuration having an automatic focusing function have been conventionally provided. For instance, when information is recorded on an optical card and is reproduced therefrom, the arrangement shown in FIG. 6 is often used because of simple structure and excellent reproducing operation, etc.
For reproducing information recorded on an optical card using an optical head shown in FIG. 6, a projected light 2 from a light source 1, e.g., a light emitting diode or a laser diode, etc. is changed to light beams 4 through a collimating lens 3 to focus the light beams 4 using a focusing lens 5, thereafter to irradiate rays of a focused light 6 onto an optical card 7. Rays of a reflected light 8 from the optical card 7 are subjected to intensity change in accordance with the information recorded on the optical card 7 to change to beams 9 via the focusing lens 5. The beams 9 are reflected by a mirror 10 and are then converged by a lens 11, thus to reach a light receiving element 12, at which the optical intensity change is converted to a current change. Namely, the reproduction of the recorded information is conducted.
For allowing the optical head shown in FIG. 6 to have an automatic focusing function, a focus control signal must be detected. Such a detection is carried out by the method shown in FIG. 7.
FIG. 7 shows the case that the optical card 7 moves close to the focusing lens 5 to reach the position of the optical card 7.sub.1, or away from the focusing lens 5 to reach the position of the optical card 7.sub.2 by the plane vibration produced when the optical card 7 is moved in a plane direction, i.e., in directions indicated by an arrow 13 in order to reproduce the information recorded on the optical card 7.
In FIG. 7, where the optical beam 4 is focused via an optical path at the left of the focusing lens 5 and is then reflected by the optical card 7, the reflected light advances via an optical path at the right of the focusing lens 5 to change to the light beam 9, thus to reach the central portion of the light receiving element 12 via the mirror 10 and the lens 11. Further, where the light beam 4 is reflected at the position of the optical card 7.sub.1, the reflected light changes to a light beam 9.sub.1 to reach the portion below the light receiving element 12. In addition, where the light beam 4 is reflected at the position of the optical card 7.sub.2, it changes to a light beam 9.sub.2 to reach the portion above the light receiving element 12.
The light receiving element 12 has a configuration, for example, as shown in FIG. 8 such that it is divided into elements 12.sub.1 and 12.sub.2 by a divisional line 12.sub.3. These divisional elements 12.sub.1 and 12.sub.2 are arranged so that the light beam 9 is irradiated equally to the elements 12.sub.1 and 12.sub.2. Thus, the light beam 9.sub.1 is irradiated mainly to the element 12.sub.1 and the light beam 9.sub.2 is irradiated mainly to the element 12.
By applying electrical outputs 14.sub.1 and 14.sub.2 from the elements 12.sub.1 and 12.sub.2 to a differential amplifier 15, the focus control signal is taken out as an output 16 of the differential amplifier 15.
The focus control signal output 16 is obtained relatively with ease by using the above-mentioned method. However, this method has the drawback that even when the medium is located at the position in focus as indicated by the optical card 7 in FIG. 7, a focus control signal is erroneously produced if the medium has a tendency due to the inclination of the medium itself, curvature of the medium, or the like. Namely, this is because when the light beam 4 is reflected by an inclined optical card 7.sub.3 as shown in FIG. 9, it changes to a light beam 9.sub.3 to reach the portion below the light receiving element. Further, when the detection method in FIG. 8 is taken as an example, this is because light beams 9, 9.sub.1 and 9.sub.3 move toward the bottom portion in the figure, i.e., in a direction indicated by an arrow 14, respectively. It is to be noted that when the optical card 7 is inclined in a direction opposite to the optical card 7.sub.3, the light beams 9, 9.sub.1 and 9.sub.2 move in a direction opposite to the above.
As just described above, an interference focus control signal output produced by the inclination of the medium is superimposed on a normal focus control signal output produced by the plane vibration of the medium, whereby the focusing function is degraded, so that the information is impossible to record or reproduce.